This invention relates generally to fuel injection timing control and, more particularly, to step timing control of injection in hydromechanically controlled diesel engines.
Economical and yet reliable systems for adjusting injection timing automatically have long been of interest to diesel engine manufacturers as a way to help achieve acceptable emission levels, fuel economy and power as engine conditions change. Although normal, or relatively retarded, timing is appropriate for a range of engine operating conditions including medium and heavy load conditions, it results in incomplete combustion during idling and light load conditions because of relatively low cylinder pressure under such conditions, the cylinder pressure being a function of the amount of air and fuel as well as the timing of injection. Normal timing leaves a relatively short length of time for the air and fuel to mix before the onset of combustion. Also, since fuel injection with normal timing typically starts just a few degrees before the piston reaches top dead center (TDC), most of the fuel is injected, and thus most combustion occurs, after TDC as the piston moves downward and the size of the combustion chamber correspondingly decreases. The combined conditions contribute to incomplete combustion resulting in relatively low fuel economy and relatively high hydrocarbon emissions, but with relatively low nitrogen oxide emissions due to relatively low combustion temperature.
During advanced injection timing, there is more time for mixture of air and fuel and all or most of the fuel is typically injected before TDC, while the size of the combustion chamber is still decreasing. These conditions produce higher pressure and temperature resulting in more complete combustion and thus greater fuel economy and lower hydrocarbon emissions, but also resulting in high nitrogen oxide emissions except at low engine loads when relatively little fuel is being burned.
Since the timing conditions for minimum hydrocarbon emissions generally result in increased nitrogen oxide emissions, and vice versa, a compromise is typically necessary to achieve acceptable levels of both types of emissions as well as acceptable power and fuel economy under given conditions. At idling and light loads, i.e., below approximately one-fourth of full load, it is advantageous to advance the timing, whereas during medium to high load conditions it is advantageous to retard the timing.
Various types of mechanical and hydraulic timing adjustment devices have been devised both for distributor-type fuel injection systems and for unit injector systems. U.S. Pat. No. 3,951,117 to Perr, hereby incorporated by reference, discloses an example of a hydraulic link formed within the pump portion of a pump-distributor assembly to advance injection timing as a function of a fuel pressure which is responsive to engine speed and load. This patent also discloses a hydraulic link or tappet provided for the same purpose within a unit injector, i.e., an injector combining a cam-actuated pump and an injection nozzle in a single unit. In both cases the fuel itself is used as the timing fluid. The fuel supply system is a hydromechanical system including a centrifugally controlled engine speed governor and a pressure regulator. The governor establishes minimum and maximum engine speeds between which the regulator regulates the fuel pressure to the throttle as a function of engine speed. The throttle then controls the pressure of the fuel to be metered into the injectors, and the amount metered is proportionate to that fuel pressure and the metering time in accordance with the pressure-time (PT) principle.
Timing in the system of U.S. Pat. No. 3,951,117 is controlled with two control valves in a line supplying fuel to the timing chamber within which the hydraulic link is formed. One valve varies the pressure in the line as a function of engine speed; the other varies the pressure in the line as a function of load as represented by throttle position, which is considered representative of engine load because the throttle is normally manually adjusted to increase fuel pressure, and thus the quantity of fuel injected per cycle, as the load on the engine increases. The hydraulic link is variable in length as a function of the supply line pressure, and it changes the injection timing by adding to the length of a cam-actuated plunger within the pump in the pump-distributor assembly, or within the pump in the unit injector. The hydraulic link thereby changes the effective profile of the cam. Injection timing is relatively advanced with a lengthened hydraulic link and relatively retarded with a collapsed or shortened hydraulic link.
It is also known to vary injection timing mechanically, as mentioned above and as illustrated by the adjustable timing mechanism disclosed in U.S. Pat. No. 4,206,734 to Perr et al. The mechanism is designed for use with unit injectors without a hydraulic tappet but including the conventional plunger driven by a cam via a push rod, rocker arm and connecting link to the plunger. The mechanism adjusts injection timing by moving the cam end of each push rod with respect to the associated cam profile such that cam action begins earlier or later as desired.
Step timing control (STC) is a form of control in which timing adjustments are made in discrete steps rather than continuously, and it is known to have certain advantages including relative simplicity, low cost, and reliability. The Cummins PT STC unit injector system is a well-established example of an STC system providing hydraulic variable timing, having been successfully used for years with Cummins L10, M11, NT, N14 and K series engines, among others, in a variety of applications. The system provides two-step timing control with a dual-state hydromechanical control valve, and in particular a spool valve, that is actuated by fuel pressure and, when open, supplies oil from the engine lubrication system to hydraulic tappets in the injectors. The valve state is determined solely by the level of the fuel rail pressure with respect to a single predetermined threshold or switch point. The general operating characteristic of the STC valve in relation to engine load and the corresponding pressure and timing states is set forth in the following table:
Engine Load Condition Fuel Pressure STC Valve Timing Starting and light load Below threshold Open Advanced Medium to high load Above threshold Closed Normal
It is also known to implement this function with a fuel pressure switch actuating a solenoid valve, as described in the Cummins Engine Company service bulletin entitled Hydraulic Variable Timing Familiarization. Also, as described in U.S. Pat. No. 5,411,003 to Eberhard et al., the switch point of a spool valve in an STC system of the type described above can be varied as a function of temperature-related variations in the viscosity of the oil supplied to the valve. The switch point is made variable by modifying the control valve to receive an assist pressure from a viscosity-sensitive pressure divider. An STC system with continuous speed-sensitive variation of the fuel pressure threshold has also been proposed, as disclosed in U.S. Pat. No. 4,909,219 to Perr et al. The disclosed system provides stepwise adjustment of timing as a function of both engine speed and load. Within a range of engine speeds, the fuel pressure threshold for a timing change varies continuously with engine speed. The system uses a hydromechanical fuel control circuit including a hydraulic servomechanism for timing adjustment, and its operating characteristics cannot be changed without disassembly.
While the Cummins PT STC system described above is a relatively uncomplicated system of timing control and has proven reliable in numerous applications, it has been found to be susceptible to problems such as excessive black smoke production and/or poor fuel economy in certain highly loaded, cyclical applications. More specifically, such problems have been observed in excavators and some other construction or industrial equipment in which a diesel engine is operated at rated speed to drive a hydraulic pump or a generator set, for example, which experiences frequent, substantial variations in load. Such problems have been alleviated by replacing the fuel injectors as needed. However, a solution is needed that gives the injectors a longer useful life.
There is also a need for a solution that complements existing hydromechanical fuel supply systems with minimal modifications.